1. Field of the Invention
The present invention relates to a method of inspecting holes such as contact holes or via holes formed during fabrication processes of semiconductor devices such as ICs and LSIs by the use of a charged-particle beam to examine how the holes are formed.
2. Description of the Related Art
A semiconductor device is fabricated by forming plural layers on a silicon wafer (silicon substrate) for example. In this multilayer structure, an insulator layer is formed between certain layers. Contact holes or via holes are formed in this insulator layer. Conductive interconnects made of a conducting material are buried in the contact holes or via holes, whereby the layers are electrically connected.
Contact holes, for example, are formed by applying a resist on the insulator film, exposing the resist by light according to a contact hole pattern, and then performing a developing step and an etching step during fabrication of the semiconductor device.
During the formation of such contact holes, if the formed contact holes do not exactly pass through the insulator layer, if some of the resist is left behind as a film in the contact holes, or if some of the insulator film is not etched but left behind as a film, then the finally manufactured semiconductor device will malfunction and be regarded as defective.
Therefore, after contact holes are formed, to know how the contact holes are formed is important in determining whether the subsequent process sequence should be carried out. Furthermore, the development step or etching step that is the previous process step is judged to be good or bad, according to the results of the inspection of the state of the formed contact holes. Also, a defect analysis of the process for forming the contact holes can be performed according to the results of the inspection of the state of the formed contact holes.
Inspection of the state of such contact holes can be carried out non destructively by electron beam irradiation from a scanning electron microscope (SEM), for example. Specifically, the electron beam is scanned across the contact holes. A secondary electron image of the contact holes is displayed on the viewing screen of a display device according to secondary electrons detected by the scanning. A person observes the image of the contact holes. In this way, it is possible to know how the contact holes are formed. This technique is described, for example, in U.S. Pat. No. 5,953,579.
In recent years, elements constituting semiconductor devices have shrunk and have been formed in multiple layers. With this trend, the diameters of contact holes have decreased, and their depths have increased. As a result, the aspect ratio (depth/diameter) of each contact hole has increased. Consequently, the efficiency at which secondary electrons from the hole may be captured has decreased greatly. For this reason, it has become more difficult to grasp the state of the inside or bottom surface of the hole correctly. In addition, it is impossible to know whether the obtained secondary electrons emanate from the opening in the contact hole, from the inner wall surface, or from the bottom. This hinders the inspection for examining how contact holes are formed.
A very large number of contact holes are present over the whole surface of one silicon wafer. If contact holes were inspected one by one, an exorbitantly long time would be necessary. Therefore, the whole wafer surface is partitioned virtually into inspection areas. One contact hole in each inspection area is inspected as a representative one. With this inspection, however, it is not certain whether results of the inspection of the contact hole represent the state of the many etched or developed contact holes existing near the inspected contact hole. Furthermore, where only the results of inspections of a relatively small number of contact holes as described above are used, it is difficult to appropriately judge whether the development step or etching step, that is, a previous process step, is good or not.
It is an object of the present invention to provide a novel method of inspecting contact holes using a charged-particle beam to permit one to examine how the contact holes are formed.
A method of inspecting holes using a charged-particle beam in accordance with the teachings of the present invention starts with irradiating a sample substrate provided with numerous holes with a charged-particle beam. The state of the holes is inspected, based on a signal obtained by the irradiation of the beam. The charged-particle beam is directed to each inspection region containing holes on the sample substrate. An electrical current flowing between the sample substrate and ground at this time is detected. This sequence of steps is repeated for previously established inspection regions on the sample substrate. In this way, data about the current distribution on the sample substrate is obtained. A map based on brightness is displayed on a display device according to the obtained data about the current distribution.
Other objects and features of the invention will appear in the course of the description thereof, which follows.